1. Field of the Invention
The present invention relates to a method and apparatus for controlling the idle operation of an internal combustion engine. The method for controlling the idle operation of an internal combustion engine according to the invention is used for the control of an internal combustion engine equipped with an electronically controlled fuel injection system, particularly for controlling the rotational speed of such an engine during the periods of idling operation such as, idle running or very low speed running by controlling for example the quantity of fuel injected, the ignition timing or the quantity of air flow to the engine.
2. Description of the Prior Art
Generally, in an engine equipped with an electronic fuel injection system of the speed-density type, the quantity of fuel injected is determined in a manner that a stoichiometric air-fuel ratio is substantially attained by a basic fuel injection quantity determined by making an engine speed correction to a fuel injection quantity determined in accordance with a two-dimensional map of the engine rotational speed and intake pressure. And the thus determined basic fuel injection quantity is corrected for the engine cooling water temperature, the intake air temperature, the battery voltage, etc., thereby using it for controlling the engine. This basic fuel injection quantity is substantially determined principally on the basis of the intake pressure and the effect of the engine rotational speed is small compared with the effect of the intake pressure.
In the engine equipped with an electronic fuel injection system of the speed-density type, the occurrence of any disturbance in the engine at no load operation causes both the engine speed and intake pressure to vary and the quantity of fuel injected varies substantially in the same phase with the intake pressure. On the other hand, during the transitional period of the engine the torque developed in the engine varies due to a variation in the air-fuel ratio of the mixture within each cylinder caused by the difference in travel speed between the air and the fuel and a variation in the quantity of air-fuel mixture caused by variation of the intake pressure and this torque variation differs in phase from the intake pressure variation due to a time delay between the combustion of the injected fuel and the development of the torque.
Then, the engine equipped with a speed-density-type electronic fuel injection system includes a large capacity surge tank in the intake system and therefore a phase difference occurs between a variation in the engine rotational speed and a variation in the intake pressure. Thus, a phase difference occurs between a variation in the engine rotational speed and a variation in the torque developed in the engine. Due to this phase difference, when the engine rotational speed decreases, the torque is decreased causing a further decrease in the engine rotational speed, and conversely, when the engine rotational speed increases it results in a torque developing characteristic of the engine thus promoting the variation of the engine rotational speed, thereby causing a phenomenon in which the engine rotational speed become unstable.
To overcome this deficiency, as disclosed, for example, in a Japanese Lay-Open Patent Publication No. 57-38642 (1982), there has been known a method in which a variation of the rotational speed and a variation of the intake pressure of an engine at each interval of successive ignitions or fuel injections are detected so that on the basis of the variation rates a correction factor is determined for each of the ignition timing and the fuel injection quantity so as to suppress the variation of the torque developed in the engine.
However, this known method does not take into account the difference in phase between the variation in the engine rotational speed and the variation in the developed engine torque, and therefore the variation in the engine rotational speed during the idling operation is not suppressed satisfactorily.